This invention relates to an upper bundle cleaning, inspection, and repair system for a nuclear power plant steam generator.
Steam generators convert heat from the primary side of a nuclear power plant to steam on the secondary side so that the primary and secondary systems are kept separate. A typical generator is a vertical cylinder consisting of a large number of U-shaped tubes which extend from the floor or xe2x80x9ctube sheetxe2x80x9d of the generator. High temperature and pressure fluid from the reactor travels through the tubes giving up energy to a feedwater blanket surrounding the tubes in the generator creating steam and ultimately power when later introduced to turbines.
Steam generators were designed to last upwards of forty years but in practice such reliability figures have proven not to be the case. The problem is that sludge from particulate impurities suspended in the feed-water forms on the tubes which greatly affects the efficiency of the generator and can even cause the tubes to degrade to the point of causing fissures in the tubes. If radioactive primary fluid within the tubes seeps into the secondary side, the result can be disastrous. Plugging or otherwise servicing such fissures is time consuming and results in expensive down time during which power must be purchased from other sources at a great expense.
There are known methods for cleaning the tubes proximate the bottom of the steam generator using flexible lances and the like which clean the tubes using water under pressure, but since a typical steam generator can be thirty feet tall, it is difficult to reach the sludge at the upper levels of the tubes using water jets. So, chemical cleaning is used but there are several disadvantages. First, chemical cleaning is very expensive (from $5,000,000 to $10,000,000 per application) and requires an extended outage. Also, some corrosion of steam generator internals by the solvents used will occur during the cleaning. In addition, large quantities of hazardous, possibly radioactive waste may be generated. Disposal of this waste is very expensive. For these reasons, although many utilities have considered chemical cleaning, few plants have actually implemented chemical cleaning.
On the other hand, there are severe technical challenges faced when considering alternate cleaning methods. A typical steam generator has approximately 50,000 square feet of heat transfer area. The tube bundle is about 10 feet in diameter and 30 feet tall but the access alley in the middle of the tube bundle is only 3.5 inches wide and is interrupted by support plates approximately every 4 feet. There are flow slots through the support plates but they are very small in size, typically 2.75 by 15 inches. In addition, the access into the steam generator is limited to a six inch hand hole. Finally, inter tube gaps are only 0.406 wide or smaller.
Thus, the inherent design parameters of a typical steam generator make it difficult to incorporate water jet sludge lancing techniques at the upper tube bundles even though these techniques are adequate to clean the tubes at the level of the tube sheet at the bottom most portion of the steam generator. See, e.g. U.S. Pat. Nos. 4,700,662; 4,980,120; 4,887,555; 4,676,201; and 4,769,085. Furthermore, the crowded interior space of a steam generator makes it very difficult to inspect and/or repair the individual tubes near the upper regions of the steam generator.
It is therefore an object of this invention to provide an upper bundle steam generator cleaning, inspection, and repair system.
It is a further object of this invention to provide such an upper bundle steam generator cleaning, inspection, and repair system which facilitates cleaning the generator from the top down thereby flushing deposits downward during the cleaning process.
It is a further object of this invention to provide such an upper bundle steam generator cleaning, inspection, and repair system which eliminates the need to use chemical cleaning techniques and overcomes the disadvantages inherent in chemical cleaning or which can be used in conjunction with chemical cleaning.
It is a further object of this invention to provide such an upper bundle steam generator cleaning, inspection, and repair system which adequately cleans the upper bundles of the steam generator using water under pressure even within the close confines of the tubes of the steam generator.
It is a further object of this invention to provide such an upper bundle steam generator cleaning, inspection, and repair system which successfully delivers sufficient water energy to remove scale and also distributes this energy in an efficient manner throughout the tube bundle.
It is a further object of this invention to provide such an upper bundle steam generator cleaning, inspection, and repair system which accomplishes cleaning remotely thereby overcoming the access restrictions of the steam generator as well as reducing exposure of personnel to radiation.
It is a further object of this invention to provide such an upper bundle steam generator cleaning, inspection, and repair system which maximizes cleaning effectiveness with a minimum use of water.
It is a further object of this invention to provide such an upper bundle steam generator cleaning, inspection, and repair system which minimizes the number of equipment moves during the cleaning, inspection, and repair procedure thereby reducing cleaning and hence outage time.
It is a further object of this invention to provide such an upper bundle steam generator cleaning, inspection, and repair system which utilizes both a bulk cleaning, inspection, and repair head and a rigid lance for intertube inspection, cleaning, and repair.
It is a further object of this invention to provide such a system which has the capability to deliver inspection cameras; and drills, grippers, and welding or cutting devices and other tools even to the upper confines of the steam generator.
The invention results from the realization that even the upper bundles of a steam generator can be reliably inspected, cleaned, and repaired by deploying a telescoping or flexible arm up through the flow slots of the support plates of the steam generator; rotating the arm into place between the steam generator tubes; and deploying a tool such as a drill, grippers, or a welding or cutting device; providing number of cleaning nozzles; and/or a video camera and/or delivery and installing repair materials such as bars, brackets, or clamps to the individual tubes to be inspected, cleaned, or repaired.
This invention features an upper bundle steam generator cleaning, inspection, and repair system. There is a deployment and support device receivable within the steam generator including some means to raise and position a distal end of the device up to the upper bundles of the steam generator. There is a rotatable mechanism attached to the end of the deployment and support device and an arm attached to the rotatable mechanism. A cleaning device such as nozzles, an inspection device such as a camera, and/or one or more tools are attached to the other end of the arm.
In one embodiment, the deployment and support device includes a first boom coupled by a rotatable connector to a second boom, the first and second boom and the rotatable connector being insertable into an access port of the steam generator and into a lane separating two rows of tube members so that the second boom falls within the lane.
The rotatable mechanism preferably rotates the arm both horizontally and vertically within the steam generator. In one embodiment, the arm includes a set of telescoping members; and in another embodiment the arm is made of a flexible material. Alternatively, only the distal end of the arm may be made of the flexible material.
In another embodiment, the deployment and support device includes an elongated body feedable through an access in the steam generator shell proximate the tube sheet of the steam generator. The elongated body is flexible in one configuration to bend into position for extension up to the flow slots in the support plates of the interior of the steam generator, and yet rigid in another configuration for positioning and supporting cleaning, inspection, or tool devices up through the steam generator proximate the upper tube bundles of the steam generator. There is also some means for driving the elongated body up through the support plates and for retracting the elongated body back down through the support plates.
The elongated body may be a rigid chain, a pair of rigid chains, a number of bendable links, a number of rigid links, or a material self-biased to form a tube.